Redefining methods in a subCLASS

[Oleg]

Hi

Is it possible to redefine methods in a subclass (not an instance)? E.g. I get 
errors in

class (Show a) => Employee a where
  speak :: a -> [Char]
  speak x = "Employee: " ++ (show x)

class (Employee a) => Manager a where
  speak x = "Manager: " ++ (show x)

Thanks
Oleg